Method for machining a coated frictional contact surface made of electrically conductive material, and electrode for electrochemical machining

ABSTRACT

The invention relates to a method for machining a coated, subtantially cylindrical frictional contact surface ( 2 ) made of electrically conductive material. In said method, the frictional contact surface ( 2 ) is machined in an electrochemical manner. Also disclosed is an electrode ( 3 ) for electrochemical machining.

The invention relates to a method for machining a coated frictional contact surface made of electrically conductive material, and an electrode for electrochemical machining

From DE 103 16 919 A1 is known a method for for restoring an engine component. The piston contact surface of a cylinder to be restored is thereby reconditioned by a plasma layer which is applied by means of thermal plasma spraying, and subsequent mechanical machining of the coated piston contact surface by honing. The many elaborate and cost-intensive method steps are however disadvantageous, as for example the multiple mechanical fine machining by honing.

Based on the state of the art, it is thus the object of the invention to provide a better method for machining a coated, substantially cylindrical frictional contact surface.

The object with regard to the method for machining a coated frictional contact surface to be specified is solved by the characteristics of claim 1. An electrode for carrying out the method according to the invention is specified by the characteristics of claim 6. Further advantageous arrangements and further embodiments of the invention follow from the dependent claims and the description.

The object regarding the method to be given is solved according to the invention in that an electrochemical machining method is used for machining a coated, substantially cylindrical frictional contact surface of electrically conductive material.

It is the advantage of this invention that the substantially cylindrical frictional contact surface coated in such a manner is machined geometrically in a highly exact manner and with a substantially improved, as it is a finer surface fine design. These surface fine designs cannot be produced by means of conventional mechanical machining, or only with an extremely increased effort. A substantially higher wear-resistant frictional contact surface results thus which can tolerate substantially higher frictional forces in connection with a coating, particularly a thermally sprayed coating, of the frictional contact surface. At the same time, the machining method is substantially simplified and more economic, as several machining steps are omitted with the mechanical machining by for example machining down.

It is a further advantage of the invention that the surface roughness is smoothed by electrochemical removal of the roughness peaks and the porosity of the layer is kept simultaneously particularly with thermally coated frictional contact surfaces, where the thermal coating comprises a higher porosity and surface roughness due to the procedure. During later use of the frictional contact surface, this results in a lubricated frictional contact in that lubricant can be stored in the coating due to the porosity of the layer, which leads to considerably improved tribological properties and an increased wear resistance of the coated frictional contact surface.

By a corresponding arrangement of an electrode for the electrochemical machining, as a rod-shaped cone with an active outer surface for the machining, or alternatively a tubular electrode with an active inner surface for the machining, coated inner surfaces of a cylinder or alternatively coated outer surfaces of a shaft can be machined by means of the method according to the invention.

Sufficiently known apparatuses for the electrochemical machining are used for the electrochemical machining method. The method of the electrochemical machining (ECM—ElectroChemical Machining) or also of the further developed electrochemical machining, the so-called pulsed electrochemical machining (PECM—Pulsed ElectroChemical Machining) is thereby characterized in that no direct contact is present between the tool and the machining object during the machining. The tool and the machining object are positioned in a relatively rigid manner to one another and in a defined manner for the machining, so that the geometry of the machining tool is reproduced on the machining object during the machining. Alternatively to the rigid positioning, the machining object and the tool can also be moved relative to one another, preferably in a translatory or rotary movement. For the PECM method, it is particularly also useful to combine an oscillating movement with a translation or rotation, where the oscillation frequency is adapted to the pulse frequency of the electrochemical machining. The relative movement can simultaneously also be carried out in a clock form on the pulse frequency with PECM machining. During the machining, an electrical voltage is applied between the machining tool and the object to be machined, where the machining object is switched as an anode, and the machining tool as a cathode. For the machining, an existing slot, preferably smaller than 1 mm, is rinsed with a conventional electrolyte solution between the tool (cathode) and the object (anode). The material removal at the machining object thus takes place in an electrochemical manner, and the dissolved material is flushed from the electrolyte solution from the machining zone as metal hydroxide. The PECM method has a much lower slot width between the tool and the object, preferably a slot width of 0.01 to 0.2 mm, and possesses therefore a considerably higher machining exactness than the ECM method. It is furthermore characteristic for the PECM method that the machining current is not applied permanently, as with the ECM method, but is supplied as a pulsed current. The method of the electrochemical machining is further distinguished by a high process stability.

The form of the tool electrode is thus transferred to the electrically conductive material in a very exact and highly precise manner by means of the electrochemical machining. The form of the tool electrode has to be arranged thereby in dependence on the machining geometry to be produced. A conventional electrode assembly is however usually used, which comprises a special geometric arrangement on the geometry to be produced, for example the exact diameter of a cylinder contact surface to be produced.

Due to the contactless machining method, the tool wear of the electrode is extremely low, whereby a high reproducibility of the method is ensured.

It is furthermore advantageous, that, with the method according to the invention, during the electrochemical machining, only a minimum material removal of less than 2.5 mm takes place, preferably in the region of 0.05 mm to 0.5 mm. The material removal, that is, the removal rate during the electrochemical machining, is furthermore controlled directly via the voltage applied in the method and/or via the conductivity of the electrolyte solution, so that the efficiency of the method according to the invention can thereby be adapted by short clock cycles with a simultaneous very high surface quality of the machined surface. That is, for a higher material thickness to be removed, an electrolyte solution with higher conductivity, that is, an increased salt part has to be chosen and/or the applied voltage has to be increased. The electrochemical machining of coated frictional contact surfaces, in particular of cylinder contact bearings, will thereby also be economical for a serial production. The machining time is reduced to a clock cycle of a few seconds depending on the material removal, preferably with a material removal of 0.1 mm to below 10 s. This clock cycle can be reduced further by the parallel machining of several components.

This is increased further advantageously specially by the PECEM method with regard to the highly exact machining of the method, whereby a high surface quality in the region of surface roughnesses R_(z) smaller than 5 μm is achieved, preferably R_(z) in the region of 0.5 μm to 2 μm. A surface is produced therewith which is considerably more even and smooth and thereby comprises a higher wear resistance compared to the conventional mechanical machining.

A further advantage of the PECM method is that a highly exact and precise machining with a microstructuring of the machining surface is facilitated by a corresponding arrangement of the electrode, for example a microstructuring in the form of microlubricant pockets or specifically aligned microgooves, whereby the wear resistance and the load capacity of the frictional contact surface 15 increased further.

The coated frictional contact surfaces are machined in a defined geometric noncircular manner related to their cross section by the electrochemical machining in an advantageous arrangement.

It is an advantage hereby that the warping of the frictional contact surface in the load state due to the deformation of the frictional contact surface are reduced by the geometrically noncircular machining of the frictional contact surface by means of an electrochemical machining method. The load capacity and the wear resistance of the frictional contact surface are thereby increased further in an advantageous manner.

As such a geometrically noncircular machining geometry are thereby not to be understood rotation-symmetric geometries with regard to the geometric center of the substantially circular or annular cross section of the frictional contact surface. For example, an elliptic, that is, a machining geometry in oval form of the frictional contact is for example to be understood thereby. Such a machining cannot be produced with conventional mechanical machining at least with a justifiable effort, where this is machined in an easy manner with electrochemical machining by a corresponding arrangement of the electrode.

The advantage of the machining geometry in oval form especially with a cylinder contact surface is that it comprises a considerably more exact rotation-symmetric cylinder geometry in the load state, that is, in the deformed state due to specific acting thermal and mechanical forces. The machining in oval form ensures a cylinder contact surface comprising a significantly higher wear resistance and smoothness compared to the conventional circular mechanical machining of a cylinder contact surface, which is deformed in an unsymmetric manner in the load state. The respective arrangement of the frictional contact surface in oval form depends on the forces occurring in the load case, but the difference of the main and the secondary axis of such a machining geometry in oval form is smaller than 10 μm in its amount, preferably in the region of 1 μm to 100 μm.

The area or the areas of the load transmission in the load state on the frictional contact surface are decisive for the exact position of the noncircular machining geometry of the frictional contact surface.

It is furthermore advantageous that the electrochemical machining for electrically conductive materials is a material-independent machining method. That is, electrically conductive coatings or materials can also be machined which can only be machined insufficiently or with high costs to the final contour by mechanical machining, for example thermal sprayed coatings which are difficult to machine down, in particular on the basis of iron chrome.

Further objects of the invention and further advantageous arrangements of the solutions according to the invention are explained in more detail in the following embodiment and the figure.

FIG. 1 thereby shows a schematic depiction, not to scale, of a cross section through a cylinder of a cylinder liner (1) for use in a crankcase of an internal combustion engine and a machining electrode (3) according to the invention at the end of the electrochemical machining. The oval arrangement of the frictional contact surface (2) of the cylinder liner (1) was shown in an inflated manner for better understanding.

For the manufacture of 4 cylinder series engines for motor vehicles, cylinder liners (1) of cast iron are cast into a crankcase of aluminum. The cylinder contact surfaces of a cylinder liner (1) are coated with a wear-resistant iron chrome layer (2) via thermal spraying by means of laser wire spraying (LDS). The layer thickness of the iron chrome layer (2) is 0.5 mm. The inner diameter of a cylinder (1) to be coated is 75 mm with a height of 100 mm.

In a subsequent method step, the final machining of the cylinder contact surfaces takes place by means of PECM. The electrochemical machining takes place in a usual apparatus for the PECM machining, not described further. The connection means necessary for the machining for the reception of the electrodes (3), for the current supply, for the defined positioning of the cylinder liner (1) relative to the electrodes (3), and for the further process control are hereby not explained in more detail, but are naturally present.

For increasing the efficiency of the PECM machining, the electrochemical machining of the four cylinder contact surfaces of a cylinder liner (1) runs in parallel, for which the apparatus comprises a corresponding number of electrodes (3).

For the PECM machining of a coated cylinder contact surface (2), an electrode (3) is used which comprises a height of 110 mm and a basic oval form, where the difference of the main axis b and the secondary axis a has an amount of 10 μm and is constant over the height of the electrode (3). The basic oval form is constant over the height of the electrode (3). The complete circumferential outer surface of the electrode (3) is electrochemically active during the PECM machining, that is, it takes part in the material removal. The front surface of the electrode (3) is insulated.

The described electrodes (3) generate the desired machining geometry in oval form of the coated cylinder contact surface during the PECM machining by their special arrangement.

During the method for the PECM machining, the cylinder liner (1) is selectively received and clamped in the apparatus. The machining electrodes (3) in oval form are subsequently positioned in the individual cylinders (1) in an automated manner. A coated cylindrical contact surface (2) thereby encloses a previously described electrode (3) in oval form in such a manner, that the secondary axis a of the machining electrode (3) in oval form is vertical to the connection line of the centers of the four cylinders arranged in series. A minimum work slot of about 0.1 mm in the area between the coated cylindrical contact surface (2) and the machining electrode results therefrom, which is in the direction of the main axis b and vertical to the secondary axis a of the machining electrode (3). The electrolyte solution, a common salt solution, is introduced from above to the machining under ambient pressure, but it can also be introduced to the machining in any arbitrary manner. The PECM machining takes place with a clock cycle of 10 s.

The procedure takes place in a fully automated manner, so that the machined cylinder liner (1) is removed from the apparatus after the PECM machining in an automated manner, and a further cylinder liner (1) to be newly machined is introduced into the apparatus. 

1. A method for machining a coated, substantially cylindrical frictional contact surface (2) of electrically conductive material, said method comprising: machining the frictional contact surface (2) in an electrochemical manner.
 2. The method according to claim 1, wherein the frictional contact surface (2) is machined in a geometrically noncircular manner in its cross section by the electrochemical machining.
 3. The method according to claim 1, wherein the frictional contact surface (2) is made geometrically oval in its cross section by the electrochemical machining.
 4. The method according to claim 1, wherein the frictional contact surface (2) defined by the electrochemical machining is microstructured.
 5. The method according to claim 1, wherein, during the electrochemical machining, a machining electrode (3) and the frictional contact surface (2) to be machined are moved relative to one another.
 6. An electrode (3) for electrochemical machining, wherein the electrode is formed in a conical manner, and wherein the electrode (3) comprises an oval cross section.
 7. The electrode (3) according to claim 6, wherein the electrode (3) comprises a tubular cross section.
 8. The electrode (3) according to claim 6, wherein the area of the electrode surface, which is electrochemically active during the electrochemical machining, comprises microstructuring.
 9. The method according to claim 5, wherein the machining electrode (3) and the frictional contact surface (2) to be machined are moved relative to one another in at least one of a translatory, rotary and oscillatory manner. 